Field of the Invention
This invention relates to the field of data processing systems. More particularly, this invention relates to a technique for transferring data values via a data bus and the format with which data values are stored.
It is known to provide data processing systems with the intent that the data processing operations performed and data values processed by such systems are difficult to determine from examination of the system. A good example of such a secure system is a smart card.
Smart cards are commonly used to control access to subscription services, such as satellite television. Such smart cards have secure encryption keys stored thereupon to allow a properly authorised user to decode received satellite television signals. It is highly desirable that the encryption keys and other security measures taken to ensure that only authorised paying customers can decode the satellite television signals are maintained confidential to prevent their unauthorised use. To this end, considerable effort is expended in making it difficult to determine the data processing operations being performed and the data values being manipulated within a smart card whilst it is operating by observing the smart card.
One type of security attack that can be made upon smart cards relies upon an analysis of the power being consumed by the smart card over time as it performs its processing operations. Small variations in the power consumed occur due to differences in the processing operations being performed and the data values being processed. It is possible with careful analysis of such power consumption data to compromise the security of such smart cards. Measures which can help increase the resistance of data processing systems to such power analysis attacks and generally obscure their operation are strongly desirable.